An adaptation of the program IRREG (4) will be developed to calculate radiation doses in fields shifted off-axis by the independent collimators of medical accelerators. A validation of this concept for both 6 and 245 MeV x-ray beams and refinements to improve calculation accuracy for fields shifted off-axis will be the foci of this research. The scatter component of dose will be characterized by modified scatter-maximum ratio SMR, (3) that are functionally equivalent to scatter-air ratios. The zero-area tissue-maximum ratio, TMRO(r,d), for depth d and radial displacement r, characterizing the primary component, will be calculated from off-axis fan-line depth doses using the central axis SMR values. The adequacy of the separation of primary and scatter components will be verified by comparing measured and predicted values of dose under conditions where the relative primary and scatter contributions to the total dose are altered. Phantom scatter factors required for the modified SMR calculation will be determined by two different techniques. A technique for optimizing the zero-area extrapolation of the phantom scatter factor will be investigated. The sensitivity of the calculation to errors in the zero-area extrapolation of phantom scatter factors and central axis TMR will be determined by a perturbation analysis. The output variation across the beam aperture wil be obtained as the product of an average phantom scatter factor, a collimator function, and an unperturbed "in-air" intensity function. The latter two functions will be determined from in-phantom measurements at the normalization depth using an iterative technique to remove the residual effects of the collimator on the unperturbed intensity function. The absolute output at the normalization depth in the center of shifted fields will be determined as a function of both field size and field offset using an automated measuring technique. This function will be analyzed to permit other users to specify outputs ousing a minimum number of optimally spaced measurements. The fan-line data structure will be examined for its utility in supporting independent collimator treatment planning procedures. A Fortran program will be written to generate independent collimator beam data in a modification of the fan-line data format so as to allow conventional treatment planning systems to predict the dose distribution associated with fields shifted off-axis. A minimal data set required to support this algorithm will be defined.